An invasive neoplastic tumor of the stomach composed of intestinal-type glandular tubules (intestinal subtype of adenocarcinoma) or foveal epithelial cells (low-cohesion subtype of adenocarcinoma). Intestinal-type adenocarcinoma presents with mucus-containing vacuoles and may show calcification, endocrine cells, and rarely Paneth cells. Well-differentiated intestinal adenocarcinomas are characterized by columnar epithelium producing mucus and tubules with branched, tortuous structures; poorly differentiated lesions may exhibit solid tissue. The low-cohesion subtype of adenocarcinoma is characterized by the infiltration of single atypical cells that may contain vacuoles (signet ring cells).